Categories

Meta

Month: July 2009

This is Bärbel Hönisch with a mass spectrometer used to measure boron isotopes to reconstruct past CO2. – Lamont-Doherty Earth Observatory

Researchers have reconstructed atmospheric carbon dioxide levels over the past 2.1 million years in the sharpest detail yet, shedding new light on its role in the earth’s cycles of cooling and warming.

The study, in the June 19 issue of the journal Science, is the latest to rule out a drop in CO2 as the cause for earth’s ice ages growing longer and more intense some 850,000 years ago. But it also confirms many researchers’ suspicion that higher carbon dioxide levels coincided with warmer intervals during the study period.

The authors show that peak CO2 levels over the last 2.1 million years averaged only 280 parts per million; but today, CO2 is at 385 parts per million, or 38% higher. This finding means that researchers will need to look back further in time for an analog to modern day climate change.

In the study, Bärbel Hönisch, a geochemist at Lamont-Doherty Earth Observatory, and her colleagues reconstructed CO2 levels by analyzing the shells of single-celled plankton buried under the Atlantic Ocean, off the coast of Africa. By dating the shells and measuring their ratio of boron isotopes, they were able to estimate how much CO2 was in the air when the plankton were alive. This method allowed them to see further back than the precision records preserved in cores of polar ice, which go back only 800,000 years.

The planet has undergone cyclic ice ages for millions of years, but about 850,000 years ago, the cycles of ice grew longer and more intense-a shift that some scientists have attributed to falling CO2 levels. But the study found that CO2 was flat during this transition and unlikely to have triggered the change.

“Previous studies indicated that CO2 did not change much over the past 20 million years, but the resolution wasn’t high enough to be definitive,” said Hönisch. “This study tells us that CO2 was not the main trigger, though our data continues to suggest that greenhouse gases and global climate are intimately linked.”

The timing of the ice ages is believed to be controlled mainly by the earth’s orbit and tilt, which determines how much sunlight falls on each hemisphere. Two million years ago, the earth underwent an ice age every 41,000 years. But some time around 850,000 years ago, the cycle grew to 100,000 years, and ice sheets reached greater extents than they had in several million years-a change too great to be explained by orbital variation alone.

A global drawdown in CO2 is just one theory proposed for the transition. A second theory suggests that advancing glaciers in North America stripped away soil in Canada, causing thicker, longer lasting ice to build up on the remaining bedrock. A third theory challenges how the cycles are counted, and questions whether a transition happened at all.

The low carbon dioxide levels outlined by the study through the last 2.1 million years make modern day levels, caused by industrialization, seem even more anomalous, says Richard Alley, a glaciologist at Pennsylvania State University, who was not involved in the research.

“We know from looking at much older climate records that large and rapid increase in C02 in the past, (about 55 million years ago) caused large extinction in bottom-dwelling ocean creatures, and dissolved a lot of shells as the ocean became acidic,” he said. “We’re heading in that direction now.”

The idea to approximate past carbon dioxide levels using boron, an element released by erupting volcanoes and used in household soap, was pioneered over the last decade by the paper’s coauthor Gary Hemming, a researcher at Lamont-Doherty and Queens College. The study’s other authors are Jerry McManus, also at Lamont; David Archer at the University of Chicago; and Mark Siddall, at the University of Bristol, UK.

The band of heavy precipitation indicates the intertropical convergence zone. The new findings are based on sediment cores from lakes and lagoons on Palau, Washington, Christmas and Galapagos islands. – University of Washington

The rain band near the equator that determines the supply of freshwater to nearly a billion people throughout the tropics and subtropics has been creeping north for more than 300 years, probably because of a warmer world, according to research published in the July issue of Nature Geoscience.

If the band continues to migrate at just less than a mile (1.4 kilometers) a year, which is the average for all the years it has been moving north, then some Pacific islands near the equator – even those that currently enjoy abundant rainfall – may be drier within decades and starved of freshwater by midcentury or sooner. The prospect of additional warming because of greenhouse gases means that situation could happen even sooner.

The findings suggest “that increasing greenhouse gases could potentially shift the primary band of precipitation in the tropics with profound implications for the societies and economies that depend on it,” the article says.

“We’re talking about the most prominent rainfall feature on the planet, one that many people depend on as the source of their freshwater because there is no groundwater to speak of where they live,” says Julian Sachs, associate professor of oceanography at the University of Washington and lead author of the paper. “In addition many other people who live in the tropics but farther afield from the Pacific could be affected because this band of rain shapes atmospheric circulation patterns throughout the world.”

The band of rainfall happens at what is called the intertropical convergence zone. There, just north of the equator, trade winds from the northern and southern hemispheres collide at the same time heat pours into the atmosphere from the tropical sun. Rain clouds 30,000 feet thick in places proceed to dump as much as 13 feet (4 meters) of rain a year in some places. The band stretching across the Pacific is generally between 3 degrees and 10 degrees north of the equator depending on the time of year. It has recently been hypothesized that the intertropical convergence zone does not reside in the southern hemisphere for reasons having to do with the distribution of land masses and locations of major mountain ranges in the world, particularly the Andes mountains, that have not changed for millions of years.

The new article presents surprising evidence that the intertropical convergence zone hugged the equator some 3 ½ centuries ago during Earth’s little ice age, which lasted from 1400 to 1850.

The authors analyzed the record of rainfall in lake and lagoon sediments from four Pacific islands at or near the equator.

One of the islands they studied, Washington Island, is about 5 degrees north of the equator. Today it is at the southern edge of the intertropical convergence zone and receives nearly 10 feet (2.9 meters) of rain a year. But cores reveal a very different Washington Island in the past: It was arid, especially during the little ice age.

Among other things, the scientists looked for evidence in sediment cores of salt-tolerant microbes. On Washington Island they found that evidence in 400- to 1,000-year-old sediment underlying what is now a freshwater lake. Such organisms could only have thrived if rainfall was much reduced from today’s high levels on the island. Additional evidence for changes in rainfall were provided by ratios of hydrogen isotopes of material in the sediments that can only be explained by large changes in precipitation.

Sediment cores from Palau, which lies about 7 degrees north of the equator and in the heart of the modern convergence zone, also revealed arid conditions during the little ice age.

In contrast, the researchers present evidence that the Galapagos Islands, today an arid place on the equator in the Eastern Pacific, had a wet climate during the little ice age.

They write, “The observations of dry climates on Washington Island and Palau and a wet climate in the Galapagos between about 1420-1560/1640 provide strong evidence for an intertropical convergence zone located perennially south of Washington Island (5 degrees north) during that time and perhaps until the end of the eighteenth century.”

If the zone at that time experienced seasonal variations of 7 degrees latitude, as it does today, then during some seasons it would have extended southward to at least the equator, Sachs says. This has been inferred previously from studies of the intertropical convergence zone on or near the continents, but the new data from the Pacific Ocean region is clearer because the feature is so easy to identify there.

The remarkable southward shift in the location of the intertropical convergence zone during the little ice age cannot be explained by changes in the distribution of continents and mountain ranges because they were in the same places in the little ice age as they are now. Instead, the co-authors point out that the Earth received less solar radiation during the little ice age, about 0.1 percent less than today, and speculate that may have caused the zone to hover closer to the equator until solar radiation picked back up.

“If the intertropical convergence zone was 550 kilometers, or 5 degrees, south of its present position as recently as 1630, it must have migrated north at an average rate of 1.4 kilometers – just less than a mile – a year,” Sachs says. “Were that rate to continue, the intertropical convergence zone will be 126 kilometers – or more than 75 miles – north of its current position by the latter part of this century.

Fifty million years ago, the North and South Poles were ice-free and crocodiles roamed the Arctic. Since then, a long-term decrease in the amount of CO2 in the atmosphere has cooled the Earth. Researchers at Yale University, the Carnegie Institution of Washington and the University of Sheffield now show that land plants saved the Earth from a deep frozen fate by buffering the removal of atmospheric CO2 over the past 24 million years.

While the upper limit for atmospheric CO2 levels has been a focus for discussions of global warming and the quality of life on Earth, this study points to the dynamics that maintain the lower sustainable limits of atmospheric CO2.

Volcanic gases naturally add CO2 to the atmosphere, and over millions of years CO2 is removed by the weathering of silica-based rocks like granite and then locked up in carbonates on the floor of the world’s oceans. The more these rocks are weathered, the more CO2 is removed from the atmosphere.

“Mountain building in places like Tibet and South America during the past 25 million years created conditions that should have sucked nearly all the CO2 out of the atmosphere, throwing the Earth into a deep freeze,” said senior author Mark Pagani, associate professor of geology and geophysics and a member of the Yale Climate and Energy Institute’s executive committee. “But as the CO2 concentration of Earth’s atmosphere decreased to about 200 to 250 parts per million, CO2 levels stabilized.”

The study, published in the XX issue of Nature, looked for a possible explanation They used simulations of the global carbon cycle and observations from plant growth experiments to show that as atmospheric CO2 concentrations began to drop towards near-starvation levels for land plants, the capacity of plants and vegetation to weather silicate rocks greatly diminished, slowing the draw-down of atmospheric CO2.

“When CO2 levels become suffocatingly low, plant growth is compromised and the health of forest ecosystems suffer,” said Pagani. “When this happens, plants can no longer help remove CO2 from the atmosphere faster than volcanoes and other sources can supply it.”

“Ultimately, we owe another large debt to plants” said co-author Ken Caldeira from the Carnegie Institution of Washington at Stanford University. “Aside from providing zesty dishes like eggplant parmesan, plants have also stabilized Earth’s climate by inhibiting critically low levels of CO2 that would have thrown Earth spinning into space like a frozen ice ball.”

Co-author David Beerling from Sheffield University adds, “Our research supports the emerging view that plants should be recognized as a geologic force of nature, with important consequences for all life on Earth”

There has never been so little sea ice in the area between Svalbard and Greenland in the last 800 years. – NASA/GSFC.

New research, which reconstructs the extent of ice in the sea between Greenland and Svalbard from the 13th century to the present indicates that there has never been so little sea ice as there is now. The research results from the Niels Bohr Institute, among others, are published in the scientific journal, Climate Dynamics.

There are of course neither satellite images nor instrumental records of the climate all the way back to the 13th century, but nature has its own ‘archive’ of the climate in both ice cores and the annual growth rings of trees and we humans have made records of a great many things over the years – such as observations in the log books of ships and in harbour records. Piece all of the information together and you get a picture of how much sea ice there has been throughout time.

Modern research and historic records

“We have combined information about the climate found in ice cores from an ice cap on Svalbard and from the annual growth rings of trees in Finland and this gave us a curve of the past climate” explains Aslak Grinsted, geophysicist with the Centre for Ice and Climate at the Niels Bohr Institute at the University of Copenhagen.

In order to determine how much sea ice there has been, the researchers needed to turn to data from the logbooks of ships, which whalers and fisherman kept of their expeditions to the boundary of the sea ice. The ship logbooks are very precise and go all the way back to the 16th century. They relate at which geographical position the ice was found. Another source of information about the ice are records from harbours in Iceland, where the severity of the winters have been recorded since the end of the 18th century.

By combining the curve of the climate with the actual historical records of the distribution of the ice, researchers have been able to reconstruct the extent of the sea ice all the way back to the 13th century. Even though the 13th century was a warm period, the calculations show that there has never been so little sea ice as in the 20th century.

In the middle of the 17th century there was also a sharp decline in sea ice, but it lastet only a very brief period. The greatest cover of sea ice was in a period around 1700-1800, which is also called the ‘Little Ice Age’.

“There was a sharp change in the ice cover at the start of the 20th century,” explains Aslak Grinsted. He explains, that the ice shrank by 300.000 km2 in the space of ten years from 1910-1920. So you can see that there have been sudden changes throughout time, but here during the last few years we have had some record years with very little ice extent.

“We see that the sea ice is shrinking to a level which has not been seen in more than 800 years”, concludes Aslak Grinsted.

During the past decade, residents of Pasto, Colombia, and neighboring villages near Galeras, Colombia’s most dangerous volcano, have been threatened with evacuation, but compliance varies. With each new eruption — the most recent explosion occurred June 7-9 — Colombian officials have grown increasingly concerned about the safety of the residents who live within striking distance of Galeras, located 700 km from Bogota.

Now, geologists from the University at Buffalo and the Universidad de Nariño have organized a special workshop in Colombia designed to tackle the communication issue, with support from the National Science Foundation and the Universidad de Nariño.

The purpose is to develop a consensus as to how best to raise awareness and protect these communities from dangerous eruptions at Galeras.

Unlike most scientific workshops, which are exclusively attended by scientists, this program will include the active participation of local residents and government officials working together with the scientists in all of the workshop sessions.

From July 6-11, Michael F. Sheridan, Ph.D., an internationally renowned volcanologist and director of UB’s Center for Geohazards Studies, and Gustavo Cordoba, Ph.D., a post-doctoral researcher in the UB center, will run the workshop on “Knowledge Sharing and Collaboration in Volcanic Risk Mitigation at Galeras Volcano, Colombia.” Complete information is at http://galerasworkshop09.weebly.com/index.html.

The first half of the workshop, which will feature professors from the UB Department of Geology, the Universidad de Nariño in Colombia, officials from the local and federal government and the Red Cross, among others, will cover the history of volcanic eruptions at Galeras, volcanic crisis management, the physics and modeling of explosive volcanism and discussions about crisis management at Soufriere Hills Volcano, Chaiten Volcano,Vesuvius and others.

The second half of the workshop will begin July 10 with a session called “The People Speak.”

Sheridan said that this part of the workshop puts a spotlight on the critical connection between local populations affected by an adjacent hazard and the level of scientific understanding and certainty — or the lack of it — about that hazard.

“The villagers feel they are safe,” said Sheridan.

In one example, he said, some of them have said that there is a sacred stone with petroglyphs on it that lies directly in the path where volcanic debris is expected to flow, but it has been there for 500 years and has never been damaged by eruptions at Galeras.

The workshop will use the example of a bridge that connects a village in the region (La Florida) to the capitol city Pasto, a city of 400,000 located only six miles from the crater of Galeras.

“Using our computational tools, we will show that if mudflows from this volcano inundate the bridge, then the evacuation route will be gone,” he said.

At the workshop, scientists, officials and residents will analyze existing hazard maps and safety plans for Galeras in light of the latest research on forecasting volcanic hazards.

“Our hope is that through the presentations by scientists and crisis management experts about what has happened at other volcanoes, and by using some visual tools, like computational modeling of mud and debris flows, we can help people living around the volcano better understand the hazard they live with,” said Sheridan.

With decades of experience all over the globe, working with scientists, governments and local populations, Sheridan concedes that it will be a challenge to try to improve the residents’ preparedness by attempting to better communicate how vulnerable they may be to eruptions at Galeras.

Still, he says that that goal will ultimately ease the job of volcanologists and others involved with risk mitigation.

“I’d like to see the workshop end with a new approach to hazards that includes the opinions of the people who are actually living in the hazard location,” he said. “It may be too much to hope for, but if it’s possible to get them to buy into the safety plan, that would be the best outcome.”